Meet Archaeopteryx, one of the most primitive of all birds. It beautifully illustrates the transition between small predatory dinosaurs and their bird descendants. It has toothed jaws and three clawed fingers on each hand, but it also had broad wings with well-developed flight feathers. Like those of modern birds, these feathers had two asymmetrical vanes coming off a central shaft or ‘rachis’.

But despite this striking resemblance, Archaeopteryx’sfeathers differed from those of modern birds in a critical way. Robert Nudds from the University of Manchester and Gareth Dyke from University College Dublin have found that they were thinner and weaker than today’s feathers. If this early bird had tried the same flapping flight that its descendants do so effortlessly, its feathers would have buckled under the stress. It seems that this pioneer among birds wasn’t a very good flier.

Since its discovery, palaeontologists have argued about Archaeopteryx’s flying abilities. Sure, the animal had broad wings and sophisticated feathers. Its skull had an inner ear that resembled that of modern birds, suggesting that it had the coordination necessary for flight. It also lacked the bony breastbone that the larger flight muscles of modern birds attach to but those muscles could equally have attached elsewhere. Perhaps more compellingly, its shoulder joint would have prevented it from lifting its wings far enough to carry out a full upstroke.

Enter Nudds and Dyke. They studied the feathers of two of the earliest birds – Archaeopteryx, which lived around 145 million years ago in the Late Jurassic, and Confuciusornis, which lived 120 million years ago in the Early Cretaceous.

Compared to modern birds of a similar size, both prehistoric animals had primary feathers of the same length, around 20cm for Confuciusornis and 13cm for Archaeopteryx. However, their rachises –the central shaft of the feathers – were unusually narrow for their size. It’s a small difference, but enough to seriously compromise the endurance of the feathers.

Bird flight feathers have hollow rachises, and if you take two hollow cylinders with walls of the same thickness, the wider one will withstand larger forces before buckling. So it is with the ancient birds. In fact, it would have taken around ten times less force to buckle Archaeopteryx’s primary feathers than that of a living bird of the same size, and a hundred times less force to cripple Confuciusornis’s plumes.

These ancient feathers simply couldn’t have withstood the forces needed to sustain flapping flight, much less perform any manoeuvres. They could only have worked if the feathers had a fundamentally different structure to those of modern birds – if they were solid cylinders without hollow cores. Even then, they would be considerably weaker than today’s feathers.

To illustrate their point, Nudds and Dyke calculated “safety factors” for a variety of feathers. This represents the force required to buckle the feather divided by the force needed to keep the bird aloft. Modern birds as diverse as gulls, pigeons and albatrosses had safety factors ranging from 10 to 14. In contrast, Archaeopteryx and Confuciusornis had safety factors of 4 and 3, and those are generous estimates that assume the feathers were solid cylinders.

Their conclusion is clear – both primitive birds were more gliders than fliers. They could well have produced some thrust with the odd flap or so, but the “vigorous flapping flight” of modern birds was out of the question. The wing-beats that hold our feathered friends aloft evolved after the first birds did.

I wasn’t even aware of this paper. Certainly makes sense when you think about it, and it’s a strange thing to consider. Archaeopteryx tries to flap, and its feathers snap at their base. It’s looking more and more like powered, sustainable flight is unique to “crown-group” birds. I guess the next step is to look at the rachi of enantiornithines.

I don’t want to believe this, and we all know how that goes. I’m waiting for someone to disprove it. In the meantime I’m thinking there could have been cross-braces inside the feather stems (rachi), or marrow. And powered, sustainable flight is a problematic concept. A chicken is a bird I know, and it flies up to low branches to roost or to get away from predators. It is said the chicken is too heavy to fly. Not sure about that, may be other reasons. Wild turkeys can fly quite well but they don’t migrate. Then, there are other ways to fly on wings than bird flight motion, moth or beetle for example. My bias tells me to wait. Meanwhile it’s a nice paper and apparently summarized well.

Huh! I’m a little surprised at your choice of title, but it sounds like the paper takes this tack as well. So let’s compare ancient birds to some of the best fliers in the bird world.

First, pigeons and albatrosses are really remarkably good fliers, and gulls and albatrosses operate in some extremely unforgiving conditions; if they’d chosen birds that don’t deal with as much severe weather or extreme speed (people often just don’t appreciate just how well pigeons fly) you might see very different results, since those are exactly the conditions that are likely to require the high failure margins they point out.

On solid-core, it seems pretty unremarkable to me that feathers may have developed over time to optimize weight-strength ratio. With the higher weight to strength ratio in solid core, the ecological payoff for reducing weight and operating near the failure threshold is a lot greater. You rarely if ever see a modern bird feather broken at the rachis, unless they’ve been in some way crushed, which may indicate that this is not a big problem anymore. Developing hollow-core feathers may have partly removed this constraint, and limited the degree to which you can actually compare the failure criteria.

They also may be underestimating the degree to which flying mechanics can vary load. A good chunk of that high loading at the extremes of the sweep can be avoided with a number of small changes, such as having larger wing area (as I’ve generally seen Archaeopteryx illustrated) and a slightly less efficient, but safer, flap, with lower peak accelerations. Again, a trade-off that results in slightly lower performance, but not a lot.

Another important point is that the bending failure of a hollow structure versus a solid one is very different; I’d have to read the paper to see what exactly how they quantified buckling, but solid-core materials approach failure gracefully, rather than suddenly and somewhat unpredictably coming to a catastrophic failure as hollow-core things will. Think of a straw versus, well, pretty much anything else that bends. Most other things aren’t destroyed if they bend slightly into plastic deformation, but a hollow structure almost certainly will be. Solid-core allows you to actually use that region near the buckling force much more safely, which may be yet another reason why the birds they checked with hollow-core feathers are effectively staying well away from it. When you can safely bend, which hollow-core structures are quite bad at, it has major aerodynamic consequences, and should reduces the peak force on the feathers significantly.

Oh, wow, went on a lot longer than I’d intended to. Long story short:
-They probably had solid-core feathers, which is interesting and likely primitive to birds
-This means they were not the most spectacular fliers, but doesn’t tell us much past that
-Behavioral and environmental factors for the test birds weight the comparison toward a relative appearance of weakness for the ancient birds

These birds likely weren’t the equal of some of today’s highest-performance fliers, but you don’t have to be a “poor” flier to fall out of that category.

You say that muscles normally attached at the breastbone could also have attached elsewhere. But then they would have to “rewire” to make modern birds where they attach at the breastbone. How likely would this be? I was always taught that “rewiring” of organs is unlikely and too big a step (e.g. larygeal nerve still curves around aorta).

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Phenomena is a gathering of spirited science writers who take delight in the new, the strange, the beautiful and awe-inspiring details of our world. Phenomena is hosted by National Geographic magazine, which invites you to join the conversation. Follow on Twitter at @natgeoscience.

Ed Yong is an award-winning British science writer. Not Exactly Rocket Science is his hub for talking about the awe-inspiring, beautiful and quirky world of science to as many people as possible.
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